Burning Pellets in a Rocket Mass Heater

After a few prototypes, a few burns, and singeing the hair off of my hands, here's my solution to burning pellets in a rocket mass heater. Burns hot and super clean! Thanks, and enjoy!

YouTube Video


-Rob T.

Love it! Great work man!

Nice ! If you made a bigger feeder how long would it burn ? Do you have time / sq ft of pellets?

It's burning 8 pounds per hour. From the various conflicting information I can find, it looks like a pound of hardwood/softwood pellets is around 8000 BTU's per hour, so it's 64,000 BTU's hour. It's actually too much heat and the vents keep opening in the greenhouse. We'll see what happens this winter when it start to get really cold!

I love your design. Do you think wood chips or sawdust would work in your feed system?

Thanks
Aaron

Nice. I have been following your aquaponics build. love it.

Aaron Esch wrote:I love your design. Do you think wood chips or sawdust would work in your feed system?

Thanks
Aaron

I' haven't tried, it but It's unlikely it would work as well. You would have to screen the chips so you don't get sticks stuck in there. The nice thing about pellets is that they are round and roll out of the hopper. The last thing you want it for the fuel to get stuck in the hopper and getting a fire up in there.

Rob Torcellini wrote:

Aaron Esch wrote:I love your design. Do you think wood chips or sawdust would work in your feed system?

Thanks
Aaron

I' haven't tried, it but It's unlikely it would work as well. You would have to screen the chips so you don't get sticks stuck in there. The nice thing about pellets is that they are round and roll out of the hopper. The last thing you want it for the fuel to get stuck in the hopper and getting a fire up in there.

Maybe not in your awesome gravity fed system but wood chips and/or sawdust might work in a retro fitted auger off a pellet stove?

Martin Seidel wrote:

Rob Torcellini wrote:

Aaron Esch wrote:I love your design. Do you think wood chips or sawdust would work in your feed system?

Thanks
Aaron

I' haven't tried, it but It's unlikely it would work as well. You would have to screen the chips so you don't get sticks stuck in there. The nice thing about pellets is that they are round and roll out of the hopper. The last thing you want it for the fuel to get stuck in the hopper and getting a fire up in there.

Maybe not in your awesome gravity fed system but wood chips and/or sawdust might work in a retro fitted auger off a pellet stove?

It's possible. try it and let me know! The problem with most wood chips is they have a very high moisture content. It takes a lot of work to get they dry enough to burn.

I was thinking the same with the moisture content. I suppose one could dry batches of fuel on or around their RMH. Better yet, join http://opensourceecology.org use the plans to build a hammer mill and pelletizer then burn it all.

My chips would be dry and not have any sticks. They would be made from pine slab wood that has seasoned so it is dry enough to burn. I have a mountain of the stuff. I have enough to fuel a rocket mass heater for about 20 years.

I know what your going to ask.

Why not just burn the slabwood in the rocket mass heater right? Well, yes that is probably what I will do but being pine I think it will burn up fast and I will have to sit there and tend the fire. I'm thinking that if I can use a large hopper full of chips I won't have to tend the fire so much. Of course then again I don't like the idea of the extra imputs of running the chipper to turn everything into chips so I will probably look at ways to pack as much wood into the feed tube as I can. I may still chip the rest of the slabwood and use it for mulch.

Aaron Esch wrote:My chips would be dry and not have any sticks. They would be made from pine slab wood that has seasoned so it is dry enough to burn. I have a mountain of the stuff. I have enough to fuel a rocket mass heater for about 20 years.

I know what your going to ask.

Why not just burn the slabwood in the rocket mass heater right? Well, yes that is probably what I will do but being pine I think it will burn up fast and I will have to sit there and tend the fire. I'm thinking that if I can use a large hopper full of chips I won't have to tend the fire so much. Of course then again I don't like the idea of the extra imputs of running the chipper to turn everything into chips so I will probably look at ways to pack as much wood into the feed tube as I can. I may still chip the rest of the slabwood and use it for mulch.

Have you ever seen the videos people that make the "chunkers"? It's basically a rotating guillotine that cuts up boards & logs into chunks an inch or two in size. I bet you could make a hopper that fed these into a RMH.

I have.... they look kinda scary to operate. Although I have operated much worse, a large buzz saw off a tractor belt with no guards. I bet that would work well for my slab wood, however I also have an upright band saw that I think would be almost as fast as the chunker and less dangerous.

I Guess I will have to try a few things and see what I get.

Quick Update: ‎"New" feed tube, stand, and hopper. Everything was made with recycled steel. It's amazing what a little paint can do! Almost looks like it was made by a pro.

Hey Rob hows the testing doing?
I need to remind Erica to chat with you about the book.

Hi Ernie. The pellet feeder is working awesome! Yesterday I did my first long-term burn in it (over 2 hours). I burned 80 pounds of pellets in 13 hours. I'm not sure if you're familiar with the greenhouse setup, but I'm heating the greenhouse with an aquaponic system in it. I have about 1200 sq feet of greenhouse building and it has around 2000 gallons of water. I burned yesterday since it was snowing and I had basically no solar gain. I was able to maintain the building temperature at 60 degrees (30 outside), and heated the water from 55 to 63. (my fish were much happier in the warmer water).

The only problem that I've had is that some of the pellets don't burn completely and they start to accumulate on the grate and don't fall through into the ash pan so it starts to clog it up a bit. I think there's some foreign matter in these pellets (rock, metal?). This started to prevent the ash from falling through and would slow down the burn a bit. About 8 hours into the burn, I quickly scraped the grate a bit to clean it off. Considering the amount of maintenance I had to do during the burn, I would call it a success! That was about $8.50 in pellets from Home Depot. For me, the time (and my back) I saved buy not splitting is well worth it! Next up, researching hammer/pellet mills!

This beast is an inferno when it's running. I was expecting it when I originally built it, but the steel grate is about about 50% of the original thickness so I'll probably make a new one out of stainless. It will last longer, but even 302 stainless isn't rated much more than 1500-1700 degrees before it starts to flake. It's also sort of strange to see some of the clay bricks glow.

HI Rob I do remember that you where heating a green house, I didn't remember that you had Aquaponics going.
Thats very good to hear the pellet feed is doing so well (I can relate to the back saving). how did the Temp hold overnight?
SS i think will be a good test but i am with you on its just not durable enough; do you think a ceramic grate would work? It would have to be a cone 10 or 15 to handle the pure heat and i dont know if that will handle the cold shock. I think they sell a Kiln shelf that is a grating that might be a thing to try.

We do need to figure out some higher temp materials soon. Erica and I are working with a fella here to get some 3100 degree clays to try and the folks we have been working with on the cast-able are trying to find us some higher temp refractories. We will let you know what we find. Also in regards to pellet build up is it possible to get the pellets and grate down in the feed a little lower so the reflected heat from the bricks will help the burn?
Yes its cool to see the bricks glow; makes for a cheery fire as it reflects off the barrel (in case you haven't done it yet polishing the barrel just above the feed will let you check the fire a bit from across the room.). it helps when you are working away from the stove and want to see how the fire is burning.

Shoot an Email to Erica when you have time so we can get you into the book as a case study and get you credited for your work. I am sure Erica will have other requirements but it would be good for her to have the info soon.

I have a couple of tricks I'm going to try to help keep the grate free of the unburnable pieces. A few different angles and positions to help keep things moving over it.

The temp usually goes down overnight. This morning it was about 45 inside, 32 outside. The majority of the heat from this RMH goes into the water and the air. Right now very little is directed into the floor. The water works well for somewhat maintaining the heat, but it can cause terrible condensation issues on the polycarbonate.

I have a friend that does a lot of ceramic work and has a kiln....maybe I'll pick her brain on high temperature materials. I would be concerned about the ceramics being too brittle, but I will admit I know nothing about them.

That's a good tip on polishing the barrel....what few hairs I have left on my head have been cooked off a few times! I'm sure you would believe that I've spent countless hours with my face down near the burn tunnel watching the fire and fine tuning the system!

I'll send a PM to Erica with my email address.

Rob T :Just 1 vote for a cover NOT sealed for your water container, if sealed vented outside ! Allen L.

allen lumley wrote:Rob T :Just 1 vote for a cover NOT sealed for your water container, if sealed vented outside ! Allen L.

I'm not heating a water tank....it's all the water in the aquaponic system...basically impossible to cover.

Rob,

You mentioned in your video taht you were having trouble with the inner chimney, and showed video of some rather crumbly stuff. What was that, and how did you correct it? I looked for another video about it and didn't find one.

That was part of the heat riser (chimney) falling apart. I used a piece of galvanized stove pipe (It's what I had laying around). The galvanized quickly burned off (do the first burns venting the area, this stuff will kill you!). The fire is so hot that it has be corroding the steel.

For now I'm ignoring the problem. I haven't pulled it apart, but I'm hoping the sections that are falling apart are keeping their shape by the rock wool insulation. It's total speculation, but I'm really hoping the rock wool has melted and healed the wounds!

I haven't done a video of it, but when I dismantle the system, I'll do an alien autopsy video on it. Then I'll assess how to repair/replace it.

Hmm.. I was planning on using stove pipe, as well. Is this inner pipe considered a consumable that needs to be replaced periodically, or is there a better way, with perhaps a thicker steel? Like Schedule 40 6" pipe or a derad oxygen cylinder with the ends cut off?

Has anyone tried lining the inside of one of these with ceramic thermal blanket? Some of that stuff is rated to 2600F or more. Fire brick would be good, but they can fail over time, too. I can't seem to find a source for firebrick that is inexpensive enough for me. Maybe I'm overthinking it.... How many firebricks do you need for a typical setup?

I just joined and figured I would jump in and contribute.

I am, among other things, a bladesmith, and built a forge a few months ago. Check out http://www.hightemptools.com/supplies-mainpage.html
This is where I bought kaowool liner and the refractory cement. In a nutshell, by coating your ceramic liner with a product called IRC-100, you can extend the life of your liner, dramatically. This coating reflects the heat back into the burn chamber, instead of allowing the liner to absorb it. This is how I reach forge welding temps, using propane (i would prefer another fuel source, but I currently live in the middle of suburbia, and require something I can use out of my garage).

I hope this helps!

Hi all,
been lurking here and other boards...well, Nate just let the cat out of the bag - so to speak.
Follow his link above to learn more about high tech.
I will be posting my progress on the appropriate thread soon, probably the "castable core" discussion.
Thanks to all who have shared here, and the other boards.

Hi all,
been lurking here and other boards...well, Nate just let the cat out of the bag - so to speak.
Follow his link above to learn more about high tech.
I will be posting my progress on the appropriate thread soon, probably the "castable core" discussion.
Thanks to all who have shared here, and the other boards.

Nate and Andor : I will be following future events closely Certainly the products we need Are out there and probably at a reasonable price!
- has anyone found IRC-100 on the site posted ?, I didn't find it !
I did a simple google search for IRC-100 and found nothing i could use ! Pyro AL

Sorry, it is ITC-100... First product on this page:
http://www.hightemptools.com/itcproducts.html

Thanks nate !

Do you just paint this stuff onto steel to protect it? How thick?

My recommendation is to use the kaowool 2" blanket, with some refractory cement "painted" onto it, followed by a coat of the ITC-100. This is how the majority of blade smiths build their forges, and many of them reach temperatures hot enough to forge weld. This will also protect the inside of the steel barrel. Based on what I've seen of the images, there must be a gap between the burn chamber and the outter barrel, so size the chamber accordingly. I am only making an educated guess, since I've not built an RSMH, yet, that the heat will be transferred better to the "seat" since it is not being lost through the chamber shell. I would recommend testing this, before installing it, since I'm a novice to the rocket stove concept.

On a side note, has anyone investigated encasing thermoelectric couplings when the mass section is built? I haven't gone too deep into this, yet, but it would seem to be an effective way of using the heat for the secondary purpose of generating electricity, as well. I am going to do some more research, on this subject, so I may start a thread outlining the concept. It will be at least two years, before I get a chance to try these theories, myself.

What about this:

Use the 6" and 10" flue pipe, and instead of perlite and ground clay, look up a mixture for firebricks using fireclay, silica, perlite and some crushed up firebrick or ceramic fibers for strength.

Then, fill the void between the pipes with the mixture and allow to cure. Since they will be encased in steel, you will need yk let them cure longer than normal to allow the water to evaporate.

Once basically dry, heat them gently by building small, short fires without the chamber (basically a regular rocket stove) then, slowly increase the intensity of the fires and wrap the outside with Kaowool to slow the cooling process.

Once you're sure it's all dry and fired, put it in use. If the steel rots away, you should still have a nice ceramic flue.

Of course, I've never built one of these, but the physics should work well if you can avoid cracking of the ceramic you make.

Chad : my plan that may change with a little more detailed information on applying ITC-100 , will be to make a Heat Riser using 6" stove pipe/ perlite protected by a little clay slip at the top lip/ and 9 or 10'' pipe on the outside ! this probably will last me a few years, or long enough for the next generation of ridged refractory one piece Heat Risers, cheap, all the insulation I need, and almost instantly ready for use. Let someone else pay to do the research ! - Pyro - Maticly yours Allen L.

Another thought would be to cast the burn chamber from refractory cement with a wire mesh form, in a mold. The chamber could then be coated with the refractory coating. This would eliminate having to use the pipe inside...

Chad Douglas wrote:Hmm.. I was planning on using stove pipe, as well. Is this inner pipe considered a consumable that needs to be replaced periodically, or is there a better way, with perhaps a thicker steel? Like Schedule 40 6" pipe or a derad oxygen cylinder with the ends cut off?

Has anyone tried lining the inside of one of these with ceramic thermal blanket? Some of that stuff is rated to 2600F or more. Fire brick would be good, but they can fail over time, too. I can't seem to find a source for firebrick that is inexpensive enough for me. Maybe I'm overthinking it.... How many firebricks do you need for a typical setup?

While refractory is definitely a good set of tools for this job, the cheapest proven way to do the heat riser is with brick (ideally firebrick, but old soft-fired building brick has also proven surprisingly durable), surrounded with perlite or rock wool (Kaowool, Durablanket, mineral wool) insulation. Used brick is often free for the hauling; firebrick we're talking about 32-40 for a typical heat riser (burn tunnel is another 26 bricks) - at $1.25 to 2.50 per brick, that's $50 to $100. And the refractory blanket we use is typically $6-10 per 2-sq-ft (24" wide and sold by the foot), we use 8.5 feet per heat riser so that's another $50-$80. With wide-laid used building brick and Canadian rock wool home insulation, you'd be talking a tank of gas maybe.

Metal is going to have a hard life in there, most will burn, slag, or de-carbon at the very least; and it's also going to likely crack out any masonry seals around it doing its thermal expansion. If you must use metal, line it with a self-supporting insulation. The cheapest form of this is $20 for a 4-cu-ft bag of perlite, plus a few dollars worth of clay (potters' scraps, ceramic, powdered mortar clay, or refractory clay of your choice; it only takes about 1/4 of a 50-lb bag, but you can usually get a bag for between $5 and $30 depending where you are and what you get).
Make the clay into clay slip / slurry about the thickness of pancake batter, pourable but starting to feel sticky. Shake-n-bake the perlite (dampen it, or use good ventilation & protection, to avoid breathing the dust) then pack the mixture into the insulation collar. The blanket's easier, but the packed perlite is self-supporting after the metal burns out. And the perlite and clay can be used right away without waiting for it to cure - though it might set better, like the refractory, if given a proper drying/curing time.

Kiln brick also works; I'd use 2600 insulative brick because we have occasionally seen some surface vitrification of perlite and that indicates we can be up around 2300 degrees. (The vitrified perlite and clay will protect the next layer for a good long time, however.) I've also seen some weird cracking in kiln brick; be sure to set it with a clay or soft-set mortar rather than a hard-set refractory, and consider wrapping or plastering it for a secondary seal.

Ceramic flue liners - haven't tried the short sections, but longer (18") sections definitely cracked within a few firings despite pretty decent insulation.

Thanks for the updates. Would love to hear if anyone else is burning pellets, briquettes, or other alternative fuels. The furthest I've gotten is throwing the occasional pits or pinecones in alongside our split wood.
Waste wood trimmings (ladder fuels, orchard suckers, and other branches) also burn great alongside split wood; no need to run them through a mechanical chipper or shredder.

-Erica W

Well, my plan was to do exactly as you suggested. Perlite and Fireclay slip, so that when the metal fails, the riser won't.

Been looking at also covering the outside with a blanket so that the riser cools more slowly to hopefully prefent (or reduce) thermal shock.

I built a small rocket stove prototype out of some fire bricks and was burning pellets with little trouble. As the OP pointed out, the feed mechanism is also somewhat fiddly. For my testing, I was just tossing a handful in periodically to get an idea of what kind of rate I would need. I discovered that too slow or too fast is death for the burn. It seems that there is a decent-sized window for rate, so coming up with a gravity-fed system should not be terribly difficult since ultra-precise regulation isn't needed.

As part of my other alternative heating explorations, I have been looking into burning waste oil. Specifically waste motor oil. It's usually quite plentiful. However, I was thinking to combine the wood pellet and oil burn into a single rocket stove. Most oil burners either use air flow to mist the oil into the burn chamber or they drip oil onto a steel plate that is in the burn chamber that gets so hot that no residue of the oil is left.

I thought of an alternative. Take some of the wood pellets and soak them in the oil until they are saturated. Some experimentation will be required to find the right ratio of oiled to non-oiled pellets, but it will allow me to get waste oil into the combustion chamber with nothing more than the gravity feed pellet system I want to use anyway. This also allows for easy switching between pure pellets and oiled/pure ratio mix or even pure oiled ones if that's doable.

I'd also like to design the gravity feed to be able to handle wood chips. Those are often readily available at certain times of year, or if someone is having a tree chipped, you could pull up in your truck with a shovel and offer to take the chips off their hands.

At any rate, I am still a newb to this, and have not het built a mass heater -- just a stacked brick (no mortar or sealant between them) rocket stove and I'm hooked. My understanding of thermodynamics (I'm no expert, but I've had some casual exposure) has given me lots of ideas on how to improve effeciency as well as design ways to burn alternate fuels. I'm doing this because my wife and I plan to move to the Texas desert and go off grid in the next couple years, and, there isn't an abundance of trees out there, let alone sticks. Since our coldest times will only dip into the 20s, we don't need a ton of heating capacity, but I could see us running the RMH to heat water or other tasks (perhaps heating oil to use in radiant baseboard heaters)

Right now, my attention has been drawn away from RMH to deal with another aspect of planning this move, but I'll circle back and once I have something that works (mostly) I'll post about it and solicit comment and suggestions for improvements.

Thanks!

A long time ago I studied burning waste oil to death and found no cheap way to do it cleanly and gave up. Either buy a big money burner and maintenance it or make a home made contraption burning it poorly and maybe dangerously while creating a bunch of pollution. Oil burners became more common it became a commodity, now a gallon of waste oil goes for .77 per gallon picked up. Then I studied making biodiesel, again all the easy free sources dried up when everyone started making biodiesel now veggie oil is a commodity too. Pellets go for around 200 per ton and thats what I am heating my shop with now, house is propane since I was not able to find a more worthwile replacement.

Hoping to be able to burn wood chips as well, I may even be able to have them delivered free or darn near from an arborist nearby. Thank you for posting your layout of your burn pit, going to use some of your ideas to try to get mine put together. I may have to dry the chips or blend chips and pellets to get them to burn.

A little design change to the grate for the pellet feeder. As expected, the steel didn't hold up...

New Grate

looks grate:)

Excellent! Cant wait to see the side by side comparison of stainless to carbon steel.